Use SIGKILL or SIGTERM for killing, not SIGABRT

[BenWiederhake]

spectrum2 uses SIGABRT to kill child processes at least in NetworkPluginServer.cpp.

This makes it very non-obvious to figure out what happened, because SIGABRT usually comes from a program-internal call to abort(), which usually comes from a failing assert(), which meant that #146 and vysheng/tgl#116 (comment) were quite an adventure, whereas SIGKILL would have immediately conveyed what is happening.

Please use SIGKILL (9) or SIGTERM (15) for this.

I won't open an issue for this discussion about timeouts (both tgp and spectrum2), so if anyone cares, open an additional issue for that.